1. Field of the Invention
The present invention relates to generators and motors, and, more particularly, to a co-rotating axial type homopolar generator/motor.
2. Description of the Related Art
Generally, when contemplating a simple generator or motor, a Faraday disk generator/motor often comes to mind. A Faraday generator comprises a circular conducting disk which is rotatable about its axis in the presence of an axial magnetic field. Electrical contacts contact the disk at various radial positions on the disk, such as the center of the disk and the radially outward edge of the disk. The Faraday generator is also sometimes referred to as a unipolar or homopolar generator as the magnetic field through which the conductive disk rotates is oriented in one direction. Placing electromagnetic theory aside for the moment, it is not uncommon for one to think of cutting a magnetic field with a conductor to result in the generation of electricity. In essence, "flux-cutting" is generalized as relative motion between the conductor and the source of the magnetic field. This relative motion is not, however, the only manner in which electrical current can be generated.
If the rotating conducting disk of the simple Faraday generator is replaced with an electrically conductive cylindrical magnet that supplies its own axially aligned magnetic field, the effect is identical. If the magnet is rotated about an axis parallel to the magnetic field and electrical contacts are placed at the axis of rotation and at the outer radius of the disk, an electric voltage is generated. This type of generator is often called a co-rotating homopolar or unipolar generator as the source of the magnetic field rotates with the conductor.
The theory and mathematics of co-rotating homopolar generators is explained in greater detail in two articles which are incorporated herein by reference: "One-piece Faraday generator: A paradoxical experiment from 1851", Crooks et al., American Journal of Physics, Vol. 46, No. 7, p. 729-731, July 1978; and "Electromagnetic Induction in Moving Systems", Corson, American Journal of Physics, Vol. 24, p. 126, 1956. As explained in Crooks et al., the co-rotating generator is not novel and is often classified as a paradox to Faraday's law. However, it is the simplification of the electromagnetic theory to the concept of "flux-cutting" that creates difficulty in comprehending the co-rotating homopolar generator.
In general, there are two types of homopolar generators and motors. The first type is the axial field type, such as the generator described above, wherein the magnetic field is axially oriented and the electric field is radially oriented. For a radial field homopolar generator or motor, the second type, the magnetic field is oriented radially and the electric potential is axially oriented. An example of a simple radial type homopolar motor is disclosed in "The radial magnetic field homopolar motor", Eagleton et al., American Journal of Physics, Vol. 56, No. 9; p. 858-859, September 1988. This radial motor is comprised of a stainless steel tube having a contrapolarized magnetic rod therein. The steel tube and the magnetic rod are supported by separate bearings so that they are able to rotate with respect to each other along the longitudinal axis of the apparatus. Two electrical contacts, which are spaced apart from each other, are operatively connected to the steel tube. By providing electrical current to the tube, the tube rotates, but the magnetic rod does not rotate. Of course, the same embodiment may be used as a generator. If the steel tube is caused to rotate, current will be generated through the conductors contacting the tube. If the magnet and the tube are rotated together, the generator is a co-rotating radial field homopolar generator.
The applicability of homopolar generators has, to a large extent, been limited. Inherently, homopolar generators generate an electrical potential having extremely high amperage. The voltage generated is a function of the speed of rotation of the conductor, the strength of the magnetic field and the radius of the conductor. Specifically, the voltage is proportional to B.multidot.W.multidot.R.sup.2 where B is the magnetic field strength, W is the rotational velocity and R is the radius. To generate higher voltages, the conductor disk is rotated at higher rpms or the radius of the conductor is increased. In many instances, a high speed of rotation results in the generation of heat, such as through the electrical contacts, which must be contained to avoid deterioration of the machine and should be minimized to increase the machine's efficiency. Using a larger disk may result in a machine of bulk, weight and overall dimensions that is unacceptable for some applications.
An example of a simple axial field homopolar generator, a Faraday generator, is disclosed in U.S. Pat. No. 3,882,366. The homopolar generator is used to control the speed of a bi-directional motor as the voltage generated by the generator is indicative of the speed and direction of rotation of the motor. The generator of U.S. Pat. No. 3,882,366 comprises a conductor disk which uniformly intersects a magnetic field that is parallel to the disk's axis of rotation. Two pairs of brushes, one pair near the center of the disk and another pair near the radially outward edge of the disk, are provided for conduction of the electricity generated therethrough. The axial field homopolar generator of U.S. Pat. No. 3,882,366 is not a co-rotating homopolar generator as the conductor rotates relative to the source of the magnetic field.
Other examples of homopolar generators wherein the conductor rotates or moves but the magnetic field source does not rotate or move are disclosed in U.S. Pat. Nos. 3,529,191, 3,465,187, 4,097,958, 4,208,600, and 3,705,995. To address the problem of generation by a homopolar generator of a low voltage, high amperage output, the generator of U.S. Pat. No. 3,465,187 uses multiple disks, electrically connected to each other to have the effect of adding the voltages derived from each disk. Specifically, two disks are rotated in opposite directions on parallel axes which intersect the magnetic field. U.S. Pat. Nos. 4,097,758, 4,208,600 and 3,705,995 each disclose a radial type homopolar generator/motor having a plurality of stacked (in relation to the axis of rotation) conductor disks which are electrically connect in series to each other to result in a higher voltage signal than is created when one conductor disk is utilized.
U.S. Pat. No. 3,669,370 discloses two generators. In one embodiment, a conventional radial homopolar generator is presented which utilizes field windings for generating a magnetic field. The field windings are mounted about the generator's stationary stator. The conductive disk of the rotor rotates perpendicularly to the generated magnetic field. In a second embodiment, the field windings are mounted about the rotor and, therefore, the field windings (the source of the magnetic field) rotate with the rotor. This co-rotating homopolar generator uses liquid metal encased with the generator to provide electrical contact with the rotating disk of the rotor.
Liquid metals, such as mercury or a sodium potassium alloy; are often used as electrical contact brushes in homopolar generators in view of the heat and velocity of the rapidly rotating rotor. Secondarily, liquid metals may assist in cooling the machine. Special precautions must be taken, however, when using liquid metals. The liquid must be compatible with the composition of the materials which it contacts, must be a good wetting agent of high conductivity, be relatively inert, have low viscosity, and have a wide temperature range over which the material remains in a liquid state without deteriorating. In addition, a recirculation system is often utilized to continually cool the machine. Thus, the use of liquid metal adds undesirable expense to manufacturing costs and to the maintenance of the generator. It is preferable to utilize a contact mechanism which is less expensive than liquid metal and which does not require any such special precautions. Consequently, it is also desirable to provide a homopolar generator wherein heat generation is minimized, not only to eliminate the requirement for expensive contacts, but also to increase the efficiency of the generator by limiting energy lost to heat generation.
Another co-rotating homopolar generator is disclosed in WIPO International Publication No. WO 82/02126. The basic elements of this generator are a rotating disk conductor having co-rotating coaxial electromagnetic coils on either side of the disk. The current generated is picked up by brushes which contact the radially outward edge of the disk and the rotor shaft. To increase the efficiency of a homopolar generator by reducing the amount of energy lost to heat, the generator of this publication uses a low reluctance magnetic return path for the magnetic flux that passes through the disk conductor. To realize this benefit, a high permeability co-rotating enclosure and a high permeability, low resistance disk is utilized. In addition, brushes of special construction are employed.
The use of electromagnetic coils to generate the magnetic field necessitates that special precautions, such as those disclosed in WIPO International Publication No. WO 82/02126, be taken to increase efficiency of the generator as the coils inherently generate heat thereby reducing the generator's efficiency. It is therefore desired to develop a homopolar generator which does not utilize electromagnetic coils to avoid the reduction in generator efficiency resulting therefrom, and also to provide a generator which amplifies the voltage generated to an improved (higher) level when compared to other homopolar generators.
In addition to the specific concerns related to homopolar generators, it is also desirable to provide a generator which may be utilized for a variety of applications. Not only must the signal generated be of a sufficient voltage and amperage for the application, the generator itself must meet the physical restraints, such as size and weight, for that application. It is desired to provide a homopolar generator whose design may be utilized to generate a wide range of voltages and which may do so while remaining within the physical restraints set forth by the particular application in which the generator is to be utilized.
It is also desired that the configurable generator be comprised of off-the-shelf components to thereby minimize manufacturing, repair and maintenance costs of the generator.